1. Field of the Invention
The present invention relates to an evaluation circuit for electronic signal transducers with current interfaces, which includes a component for converting a signal from a transducer into a signal voltage.
2. Prior Art
An evaluation circuit of this kind is particularly used with actuators and sensors which emit a current as an output signal. An evaluation circuit of this kind is frequently used in sensors which are used to detect the speed of motor vehicle wheels for evaluation in antilock control devices. In this connection, the sensor output signal oscillates between two current values depending on the signal state. Usual values of the signal current are, for example, 10 milliamperes for a low signal level and 20 milliamperes for a high signal level.
In order to convert the signal current into a signal voltage that can be evaluated by means of comparators, for example, the current is conducted through a measuring resistor. The measuring resistor is connected in series with the sensor. As a result of the series connection, the voltage that is applied by way of the sensor is reduced by the voltage drop at the measuring resistor. Consequently, a different voltage is applied by way of the sensor depending on the signal state.
Since the measuring resistor is usually dimensioned so that the voltage drop in it is 1.5 to 3 volts, the supply voltage available for the sensor drops by 1.5 to 3 volts. This can impair the function of the sensor. A reduction of the measuring resistor could actually achieve the fact that the supply voltage available for the sensor would no longer have large fluctuations, but a smaller measuring resistor supplies only very small signals. Problems in the evaluation of the signals arise as a result of this. Furthermore, the influence of incorrect couplings and similar malfunctions increases.
Furthermore the measuring resistor must be designed for a relatively high power since the full supply voltage can be connected to it in the event of a malfunction of the sensor or an error in the wiring harness (e.g. a short circuit).